銚子沖産小型ツノザメ類3種の生息水深と食性

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Three species of small deep-sea squaloid sharks, Etmopterus lucifer, E. unicolor and Centro-scyllium ritteri, were examined to study their vertical distribution and feeding habits on the basis of specimens collected from the continental shelf and upper slope off Choshi, located at the Pacific coast of Japan. E. lucifer inhabited the shallowest (mean depth range 200-300m) waters among the three species whereas E. unicolor lived in the deepest waters (mainly in 450-850m depth). E. lucifer showed an increase in depth with growth, Sex ratios by depth varied from species to species. Males always outnumbered females in E. lucifer. Stomach content analysis for E. lucifer demonstrated that small squids (mostly Watasenia scintillans) were the most important prey and that fish (mainly myctophids) were next in importance. Food items of E. unicolor con-sisted mainly of Watasenia scintillans, myctophids and Natantia (Acanthephyra sp.). Euphausiids were the main food items constituting 97% of the total number of the prey in C. ritteri. On the other hand, fishes showed high frequency of occurrence in stomach contents of C. ritteri although they were not of importance in numbers. In E. lucifer, the dietary importance of crustaceans like euphausiids decreased with size and as they grew larger, they tended to feed on squids and fishes. From the above observations food overlaps among these three species are considered to be low.